Ann: CYBG PLC - First Quarter 2019 Trading Update, page-45

I don't know if anyone is saying that there won't be 'eating into equity'. If we assume a long-run return on equity of 10%, then it's logical that in the future CYB may trade at 1x book despite a potentially bad Brexit, as per earlier posts in this thread As the stock is trading on 0.6x tangible book value (or thereabouts), the market is essentially pricing in 100% chance of up to 40% permanent loss of equity (assuming no dilutive capital raising before that point). Of course, there may be a capital raising and shareholders need to live with the possibility that there is some permanent capital loss, in exchange for significant upside if things go back to normal. There's also a small chance of permanent default/failure, as with ALL businesses.
Finally, the market is assuming that a hard Brexit will cause large credit losses to CYB, this could happen, but there's a (small) chance it may not.
In summary, it's all a game of probability, and there's many here that believe the odds are very much in our favour.